Alloys used to fabricate articles such as dental protheses must withstand highly corrosive and complex environmental conditions. The fluids bathing the oral cavity possess many components that are capable of corroding alloys based on many normally corrosion resistant elements. These components comprise both organic salts and inorganic salts such as sodium chloride and potassium thiocyanate. Various digestive enzymes, organic fluids including proteins, mucin, serum albumin, globulins, and cellular material such as leucocytes and epithelial debris also make up the oral fluids. These materials are extraordinarily corrosive to a wide cross-section of alloy materials.
Since dental alloys are intended for prolonged use in conjunction with living tissues, they must also be free of toxic effects either when in direct contact with adjacent tissue; or if ingested when the alloy components are eroded or corroded by the oral fluids.
Since the advent of modern dental practices within the last 100 years, alloys based upon the noble metals, especially gold, silver, platinum and palladium have been the preferred metals from which dental protheses have been fabricated. The reason these metals have been so widely utilized resides in their qualities of relative inertness in the oral environment, lack of toxic effects, their ability to alloy with other metals to produce products with a broad range of properties such as ductility, ease of casting and burnishing, high strength, corrosion and tarnish resistance, and the like.
The principal drawback associated with the use of the aforesaid precious metals in such alloys resides in their increasingly higher cost. The cost of these precious metals has multiplied several times over in the past several decades, and it is anticipated that these costs will continue to escalate in the coming years.
Principally because of the high cost of the precious metals, but also because of a desire for prostheses of lighter weight and greater stiffness, alloys based upon cobalt and chromium, or nickel and chromium have gained wide acceptance. These Co-Cr or Ni-Cr alloys can be prepared with a wide range of properties, however they are most universally used in partial dentures and ceramic crowns and bridges. Nonetheless, these Co-Cr and Ni-Cr alloys tend to be quite hard and stiff, are difficult to grind, shape, and burnish after casting. These difficulties present a drawback to their use and accounts, in part, for their failure to fully supplant precious metal alloys.
For dental purposes, five types of alloys are generally recognized:
Type I are soft alloys exhibiting a Brinell hardness (Vickers Hardness) in the range of 40-75 (50-90). They can be easily burnished to yield good and exact margins. Such alloys are intended for inlay restorations of the simpler non stress-bearing types.
Type II alloys are medium-hard alloys exhibiting a Brinell hardness (Vickers) in the 70-100 (90-120) range. They are difficult to burnish, but can be heat treated. They are intended for use in moderate stress situations, i.e., as three quarter crowns, abutments, pontics, full crowns, and saddles.
Type III alloys are hard, strong and the least ductile of the cast-inlay alloys. These alloys exhibit Brinell hardness (Vickers) in the range of 90-140 (120-150) and cannot be burnished, they can be heat treated, but this reduces their ductility. They are used in high-stress applications in three quarter crowns, abutments, pontics, appliance supports and precision fitting inlays.
Type IV alloys are extra hard partial denture alloys used where high strength, great hardness and stiffness are required. They exhibit Brinell (Vickers) hardness in the range of 130-200 (150 and up) and are used for cast removable partial dentures, precision-cast fixed bridges, some three quarter crowns, saddles, bars, arches, and clasps.
Ceramic compatible alloys comprise the final type of dental alloys. These alloys are characterized by an ability to form strong high precision crowns and bridges coupled with an ability to strongly bond to dental ceramic materials and to withstand rupture of the alloy-ceramic bond under high stress.
Generally speaking, only the precious metal alloys can be produced to cover the entire range of alloy types, albiet with the addition of a wide range of alloying components to modify the alloy's properties for different applications. The Co-Cr and Ni-Cr alloys can be produced in a fairly wide range of properties. However they are not useful in Type I, II or III applications or where softer, more ductile alloys are needed.
There is a continuing need for alloy systems that have the versatility of the precious metal alloys, but with a cost approaching that of the Co-Cr and Ni-Cr alloys.